The present invention relates to an attachment arm for a noise attenuating device which is intended to form part of a head stirrup or head strap or to be fitted to a protective helmet, and one end of which arm can be connected to a noise attenuating, protective ear muff, such as to enable the muff to be spring-pressed against the head of the wearer.
People who occupy excessively noisy environments are obliged to wear some form of noise attenuating device to protect the wearer against noise injury. These devices are normally fitted to a protective helmet, such as those worn in forestry work, by miners, on building sites, in dockyards, and like places. The noise attenuating devices are connected to the shell of the helmet by means of spring-biased arms. The need for such noise attenuating devices, however, is also found in other connections, where the need for a protective helmet is not found. In this case, the noise attenuating device is carried by a so-called head stirrup or strap.
Two main requirements of a satisfactory noise attenuating device are that the device will attenuate noise effectively and efficiently and that it is comfortable to wear. However, since these two requirements are essentially opposed to one another, they are difficult to realize in practice. In order to obtain good protection against intense noise, it is necessary for the noise attenuating device to be pressed hard against the wearer's head, so as to prevent the occurrence of noise leakage passages between the attenuator muff sealing ring and the wearer's head. A relatively tight abutment pressure is required in the case of head stirrups, in order to ensure that the stirrup will be held firmly on the head of the wearer. This abutment pressure, however, must not be so excessive as to render the sound attenuating device uncomfortable when worn. Thus, from the aspect of comfort, the abutment pressure should be as light as possible. This may lead, however, to poor sound attenuation and poor retention of the stirrup on the head of the wearer.
Tests have shown that if the person who occupies an environment of intense noise while wearing a noise attenuating device is obliged to remove the device for 10% of the total occupation time, the risk of injury through noise is essentially no less than if the person wore no protection at all.
The protective effect of the device will be greatly impaired, even when the device is removed for 3% of the total occupation time, while an unprotected span of as little as 1% of said total time will result in a marked decrease in the protective effect. It is possible to conclude from this that the endeavor to achieve the tightest possible abutment pressure, and thus the maximum sound attenuating effect, is counter-productive if it results in the need to remove the device at times, for the sake of comfort.
Those noise attenuating devices earlier available on the market are constructed to afford good noise attenuation and, in the case of head stirrups, also good retention with head sizes above a certain minimum size, and also good comfort, although the retention of such head stirrups is less positive when the head size is below a given maximum size. In the case of large head sizes, the noise attenuating device will often press hard against the wearer's head, resulting in a reduction in comfort, whereas in the case of small head sizes the abutment pressure is so light as to impair the noise attenuating effect of the device and to render the head stirrup less secure. The difference in abutment pressures can differ to a significant extent between small and large head sizes.
These problems are amplified when attempting to manufacture the head stirrups from a plastics material, since the abutment pressure engendered with such stirrups will decrease over the time in which the stirrup is in use and will be restored to its original value when the stirrup is allowed to relax for a given length of time. Consequently, the abutment pressure needs to be very high at the beginning of a working shift in order for sufficient holding force to remain at the end of the shift and to provide for effective noise attenuation for the duration of said shift, all to the detriment of comfort.